It has been assumed that rats, with larger brains, are smarter than mice. Scientists from Cold Spring Harbor Laboratory (CSHL) have now challenged this belief.

They compared mice and rats and found very similar levels of intelligence, a result that could have powerful implications for scientists studying complex behaviours and learning.

For more than a decade, rats have been the rodent of choice for scientists studying how the brain arrives at decisions.

They are relatively inexpensive to keep and are the subject of extensive protocols for studying cognitive function. Yet the last few years have seen an explosion in the number of genetic tools available to study their smaller cousins, mice.

These tools enable scientists to turn genes on and off within specific populations of neurons specificity that is critical to understanding how complex behaviours arise.

Many investigators have shied away from using these new tools, however, believing that mice simply are not as intelligent as rats.

"Mice have the potential to greatly accelerate our research. We didn't want to discount a very powerful option based on anecdotal evidence of their inferiority," said CSHL Professor Anthony Zador.

Zador and his team systematically compared how rats and mice learn to perform a moderately challenging auditory task and found that their performance was similar.

"This was a task that tested perceptual ability as well as adaptability, and we were very surprised to see that mice and rats performed about the same," said Santiago Jaramillo, a former postdoctoral researcher in the Zador lab who now heads his own lab at the University of Oregon.

The researchers were able to find only one difference: rats learned somewhat faster than mice. According to Zador and Jaramillo, the training protocol, which was developed and optimized specifically for rats, might account for the slight advantage.

"We've found that mice, and all the genetic tools available in them, can be used to study the neural mechanisms underlying decision-making, and they might be suitable for other cognitive tasks as well," said Zador.

The study is published in the journal Frontiers in Systems Neuroscience.